In general, separators are used for separating positive and negative electrode active materials in alkaline primary batteries. The separators require a variety of performances such as the followings:
1. preventing the internal short circuit caused by contact between the positive and negative electrode active materials,
2. retaining high electrolyte-absorbing property to generate sufficient electrogenic reactions,
3. taking the less occupancy when the separator is built into the battery to increase the amounts of positive and negative electrode active materials (for prolonging the usable time of the battery), and
4. having the durability that prevents shrinkage caused by an electrolyte such as a potassium hydroxide (KOH) solution or by an electrode agent such as manganese dioxide.
As the battery separator having the above-mentioned performances, the battery separator comprising a polyvinyl alcohol-series fiber has proposed, in view of excellent chemical resistance, hydrophilicity, and mechanical performance of the fiber. The separator comprising a polyvinyl alcohol-series fiber in combination with a cellulose-series fiber has been also proposed, in view of enhancing additionally the electrolyte-absorbing property.
For example, Japanese Patent Application Laid-Open No. 146249/1989 (JP-1-146249A, Patent Document 1) discloses a separator paper for an alkaline dry battery that comprises a synthetic fiber mainly comprising an unacetalized polyvinyl alcohol-series synthetic fiber having a finesses of not more than 0.5 denier and an acetalized polyvinyl alcohol-series synthetic fiber having a fineness of not more than 0.5 denier, and the mass ratio of both fibers is 90:10 to 10:90. This document describes that 5 to 70% by mass of a cellulose-series pulp such as a mercerized cotton linter pulp is further contained relative to the total amount of the fiber and the pulp.
Japanese Patent Application Laid-Open No. 154559/1987 (JP-62-154559A, Patent Document 2) discloses an alkaline dry battery equipped with a separator paper partially or totally comprising a synthetic fiber such as a polyvinyl alcohol-series fiber having a fineness of not more than 0.8 denier. This document describes that the separator paper further comprises a cellulose-series fiber such as a rayon fiber, an acetate fiber, a cotton linter pulp, a wood pulp or an acrylic acid pulp fiber in a proportion of 15 to 85% by mass.
Although these battery separators comprise alkali-resistance cellulose materials in a high compounding proportion to enhance the electrolyte retention capability, these separators are not practical from the viewpoint of the required standards for a separator in recent years because of the change in dimension (the shrinkage in area) caused by a strong alkaline electrolyte. For example, according to the patent document 2, the separator having the denseness and the high electrolyte retention capability is obtainable. However, because the shrinkage by the area of the separator becomes not less than 5% (after immersing in 40% KOH aqueous solution, at 80° C. for 24 hours), alkali resistance of the separator declines. Moreover, the sealing part of the separator opens on impact caused by shaking or by dropping when conveying or carting the battery, and the negative electrode gel leaks out therefrom. As a result, the internal short circuit is caused.
On the contrary, Japanese Patent Application Laid-Open No. 231746/1994 (JP-6-231746A, Patent Document 3) discloses a separator paper for an alkaline battery that has alkali resistance and comprises an organic solvent-spun rayon fiber having a Canadian standard freeness value of 0 to 500 ml, in a proportion of 20 to 90% by mass. This document describes that as an alkali-resistance synthetic fiber such as a vinylon fiber or a regenerated cellulose fiber or the like is mixed. Moreover, the document describes that as one part of the organic solvent-spun rayon fiber, a linter pulp and/or a mercerized kraft pulp may be contained in a proportion of not more than 30% by mass.
Japanese Patent Application Laid-Open No. 163024/1994 (JP-6-163024A, Patent Document 4) discloses an alkaline battery separator containing as at least part of the main fibers a fibrillated product of cellulose fiber obtained by dissolving a cellulose in a solvent and depositing the cellulose directly. This document describes that the separator comprises a polyvinyl alcohol-series fiber having a fineness of not more than 1 denier in a proportion of not less than 5% by mass relative to the main fiber.
Although these separators has excellent alkali resistance, denseness, high electrolyte retention capability, and excellent electric property, the separators themselves are lacking in so-called stiffness. The separators themselves bend on impact caused by shaking or by dropping when conveying or carting the battery, and the negative electrode gel leaks. As a result, the internal short circuit is caused.    [Patent Document 1] JP-1-146249A (Claims, page 3, lower right column, line 4 to page 4, upper left column, line 11, and Example 3)    [Patent Document 2] JP-62-154559A (Claims, page 3, lower left column, lines 12 to 17, and Examples)    [Patent Document 3] JP-6-231746A (Claims, and Example 5)    [Patent Document 4] JP-6-163024A (Claims)